Various magnetic centering structures are known. They generally rely on the use of permanent magnets, windings, ferromagnetic armatures and, usually, an electrical excitation circuit making it possible to control the magnetic fluxes generated by the windings. The role of a centering magnetic bearing is to centre a mobile body with respect to a reference body. Several motions of the mobile body with respect to the reference body are thus prevented, or controlled. If an orthogonal reference frame is considered, consisting of three axes X-X, Y-Y, and Z-Z, and centred at the centre of the device comprising the magnetic centerer, the mobile body and the reference body; the Z-Z axis constituting an axis of revolution of the device, and the X-X and Y-Y axes defining a mid-plane of the device, there then exist three translations in relation to the axes X-X, Y-Y and Z-Z and three rotations about these same three axes.
Various types of centering magnetic bearings make it possible to control the three translations.
Additionally, the mobile body retains at least one degree of freedom with respect to the reference body, generally a rotation around the Z-Z axis; in this case, the mobile body is called a rotor and the reference body a stator.
Usually, the tiltings in relation to the other axes can on the other hand only be controlled in a passive manner.
Most centering magnetic bearings therefore make it possible to control the three translations of the mobile body with respect to the reference body: in relation to one active axis and two passive axes, in relation to two active axes and one passive axis, or in relation to three active axes. However, they exhibit, in the state of the art, the drawback of significant radial bulk. Specifically, windings are wound around vertical axes, parallel to the Z-Z axis, at the periphery of the bearing; the return of these windings does not serve to generate additional magnetic flux: the presence of this return is merely a physical necessity and increases the radial bulk.
Additionally, with the aim of allowing the control of tiltings about the X-X and Y-Y axes, several technologies have been developed. Those described in French patents no2797477 and no2797478 may be cited by way of examples. These magnetic bearings, which make it possible to control tiltings in relation to the X-X and Y-Y axes, still exhibit very significant radial bulk.
Another recent technology is taught in European patent EP0724086, which presents a magnetic bearing arranged on two stages, each stage comprising windings and magnets making it possible to control distinct magnetic fluxes at the level of the air gaps situated respectively above and below the mid-plane of the magnetic bearing. This technology makes it possible to control micro-tiltings of the magnetic bearing, optionally with a significant torque. However, the drawback related to the radial bulk of the magnetic bearing is still present, the windings being wound around vertical axes of revolution. Nevertheless, the present invention borrows the idea of arranging the magnetic bearing on two stages.
To summarize, the centering magnetic bearings of the state of the art all exhibit at least the drawback of significant radial bulk.
The present invention proposes a solution making it possible to solve this drawback through the use of a particular dual-stage magnetic bearing structure, making it possible to ensure performance comparable with contemporary centering magnetic bearings, but with reduced radial bulk.